Hi,
I can't help you implement it but I can explain some of the reasons why we don't have it....

leJOS relies very heavily on the link stage to remove code that is not used and to optimise the way that constants and variables are used. The problem with dynamic code is that its use means that you do not know what classes/methods etc. will be required at run time. This means that in theory you would need to have all of the classes.jar file available, at the moment this is 636Kb in size (much bigger than the available memory on the NXT). This is just the first of the many problems that dynamic code introduces....

No the leJOS VM can not dynamically load class files, and as I described above I think you will find modifying it to do so will require a large number of changes, and may not be possible. If this is for academic project/course work, I would urge you to make sure you understand exactly how complex a task you are taking on before you attempt to make such changes. leJOS relies very heavily on the changes made to a class file during the leJOS link phase many of these changes and associated assumptions are not valid for code that is loaded dynamically...